Mikrochim. Acta 111, 119-125 (1993)
Mikrochimica
Acta
9 by Springer-Verlag 1993
Printed in Austria
2-(2-Thiazolylazo)-p-Cresol (TAC) as a Reagent
for the Spectrophotometric Determination of
Titanium (IV)
S6rgio L. C. Ferreira L*, A. C. Spinola Costa 1, and Henrique A. S. A n d r a d e 2
1 Instituto de Quimica, Universidade Federal da Bahia, Salvador, Bahia, 40210, Brasil
2 Dept. de Quimica, Pontificia Universidade Cat61ica, Rio de Janeiro, R J, 22453, Brasil
Abstract. T h e reaction between titanium(IV) and 2-(2-thiazolylazo)-p-cresol-
(TAC) in aqueous m e t h a n o l media at a p p a r e n t p H 4.0-5.6 results in a intensely
coloured complex that is stable for at least 2 h. The c o m b i n i n g ratio is 1 : 1
cation : TAC. Beer's law is obeyed up to 5.0 # g / m l titanium(IV) at 580 nm. The
a p p a r e n t m o l a r absorptivity at 580 n m is 9.82.103 1.mole-l.cm -~ and the detec-
tion limit o b t a i n e d was 5 ng/ml titanium(IV). A s p e c t r o p h o t o m e t r i c m e t h o d
for the simultaneous d e t e r m i n a t i o n of titanium and iron with T A C is proposed.
Key words: 2-(2-thiazolylazo)-p-cresol, titanium determination, simultaneous
determination.
H y d r o g e n peroxide [-1] is c o m m o n l y used for the s p e c t r o p h o t o m e t r i c determination
of titanium. The m e t h o d is simple and faMy selective, but not very sensitive (the
apparent molar absorptivity is 700 1.mole-~cm -1). 2-(2-Thiazolylazo)-p-cresol (TAC)
is an alternative, it reacts with titanium(IV), forming a 1 : 1 complex with an
a p p a r e n t molar absorptivity of 9.82.103 1.mole-l.cm -1 at 580 nm, in the presence
of h y d r o x y l a m m o n i u m chloride. Iron(II) also reacts with T A C [2, 33 to give a
complex with c o m p o s i t i o n Fe(II)-(TAC)2 and a b s o r p t i o n peaks at 740, 580 and
520 nm. It is thus possible to determine titanium(IV) and iron(II) spectrophoto-
metrically with T A C in a single sample.
T A C has also been used for the s p e c t r o p h o t o m e t r i c d e t e r m i n a t i o n of copper(II)
[-4], nickel [53, b i s m u t h [6], a n d zirconium [-73.
Experimental
Reagents
TAC Solution: 0.1 g dissolved in 100 mL of methanol.
Standard titanium solution [-8]: prepared by heating 0.20 g of titanium(IV) oxide with 8 ml of concentrated sulphuric acid and 3.2 g of ammonium sulphate until dissolved, cooling, and diluting to 1 litre with water.
Standard iron(II) solution: prepared by dissolving 0.176 g ofiron(II) ammonium sulphate in 0.5% (v/v) sulphuric acid.
Buffer solution: prepared by mixing 2.0 M sodium acetate and 2.0 M acetic acid in appropriate ratio to give pH 5.
Stock solutions of others elements: prepared by dissolving suitable salts in water, 1% (v/v) nitric acid or 1% (v/v) hydrochloric acid.
Hydroxylammonium chloride solution: prepared by dissolving 10 g of the salt in 100 ml of water.
Procedure for Titanium in the Absence of Iron
Into a 25-ml standard flask, transfer a portion of solution containing up to 125/tg of titanium(IV). Add 2.5 ml of hydroxylammonium chloride solution, 8 ml of methanol, 2 ml of TAC solution and 5 ml of acetate buffer. Dilute to the mark with water, mix and after 5 min measure the absorbance at 580 nm in a 1-cm cell, using an appropriate blank. Prepare a calibration graph covering the range up to 125 Fg of titanium per 25 ml.
Procedure for Iron in the Absence of Titanium
To a 25-ml standard flask containing up to 100/~g of iron, add 2.5 ml of hydroxylammonium chloride solution, 8 ml of methanol, 2 ml of TAC solution and 5 ml of acetate buffer. Dilute to the mark with water, mix, and measure the absorbance at 580 or 740 nm using a blank. Prepare a calibration graph covering the range up to 100 #g of iron per 25 ml.
Procedure for Determination of Both Titanium and Iron in Geological Matrices
Sample Decomposition. Weigh 0.2-0.3 g of sample (dried at 110~ transfer it into a Teflon beaker, add a few ml of water, 1 ml of concentrated sulphuric acid and 10 ml of concentrated hydrofluoric acid, and heat on a hot-plate until white fumes appear. Cool, add 5-10 ml of concentrated hydrochloric acid and transfer the contents of the beaker to a 25-ml glass beaker. Heat until all the residue has dissolved, cool to room temperature, and transfer the solution into a convenient size of standard flask; make up to volume with demineralized water.
Analysis. Pipette a suitable volume of the sample solution into a 250-ml beaker, and heat it on a hot-plate almost to dryness. Cool, add 2.5 ml of hydroxylammonium chloride solution and transfer the solution to a 25-ml standard flask. Add 8 ml of methanol, 2 ml of TAC solution and 5 ml of acetate buffer. Measure the absorbances at 740 and 580 nm, against a reagent blank run through the whole procedure.
Calculation Method for Simultaneous Determination of Titanium and Iron
1. Method (a)
A'74o = eV]o" b" C v~
Ass o = eV~0.b-C ve + e ' ~ 0 - b - C xi
2-(2-Thiazolylazo)-p-Cresol (TAC)
2. Method (b)
A _ 7 4 0 =
ev40Fe, b" C Fe _~_ ev~0Ti " b" C Ti
A_580 ~-- e580Fe . b" C Fe _I_ e58oTi ' b" C Ti
This method consider the weak absorption of the titanium-TAC complex at 740 nm.
121
Results and D i s c u s s i o n
Solubility of TAC
TAC is only slightly soluble in water but readily soluble in aqueous media contain-
ing 20% or more methanol, ethanol, 2-propanol, acetone, dioxan or ethylene glycol.
Methanol, ethanol and ethylene glycol are preferred as the absorbance of the
titanium complex is higher in these media and remains stable for at least one hour.
Reaction Conditions
M a x i m a l and constant absorbance is obtained for 50 #g of titanium(IV) with
0.70 ml of 0.1% T A C solution per 25 ml; so, 2.00 ml of TAC solution was selected
as optimal.
The complex f o r m a t i o n is greatly influenced by the pH, Table 1 shows that the
best pH range is 4.0-5.6. The absorbance is somewhat dependent on the buffer
c o n c e n t r a t i o n (shown in Table 2), it is necessary to use the same buffer concen-
tration for all samples, standards and blanks.
The order of addition was studied and the results d e m o n s t r a t e d that the complex
was affected with it. The results showed (Table 3) that the reductant must be always
added before the buffer.
A preliminary experiment showed that the Ti(IV)-TAC system is not stable in
the absence of h y d r o x y l a m m o n i u m chloride (Table 4).
Table 5 shows that with titanium(IV) concentration of 2.0 #g/ml, the absorbance
of the system is not affected by the presence of from 1 to 6 ml of a 10% solution of
Table 1.
Effect of pH on complex formation.
(Ti(IV): 2.00/~g/ml)
pH
Absorbance
3.00
0.047
3.46
0.105
3.78
0.364
4.00
0.382
4.56
0.402
5.13
0.396
5.64
0.383
5.81
0.372
6.23
0.366
Table
2. Effect of the concentration
buffer on the TI(IV)-TAC system.
(Ti(IV): 2.00/~g/ml; pH 5.0)
Buffer a c e t a t e
Absorbance
at
concentration(M)
580 nm
0.10
0.416
0.20
0.405
0.30
0.391
0.40
0.377
0.50
0.365
Table 3. Effect of order of addition or on the formation of the Ti(IV)-TAC complex. (Ti(IV): 2.00 #g/ml)
Order of addition Absorbance
Titanium + Chloride" + Alcohol + TAC + Buffer + Water 0 . 4 0 5 b
Titanium + Chloride + Buffer + Alcohol + TAC + Water 0.403 Titanium + Chloride + TAC + Alcohol + Buffer + Water 0.405 Titanium + Chloride + TAC + Buffer + Alcohol + Water 0.406 Titanium + TAC + Chloride + Buffer + Alcohol + Water 0.404 Titanium + Alcohol + Chloride + TAC + Buffer + Water 0.405 Titanium + Buffer + Chloride + TAC + Alcohol + Water 0.390 Titanium + TAC + Alcohol + Buffer + Chloride + Water 0.339 Titanium + TAC + Buffer + Chloride + Alcohol + Water 0.396 Titanium + Buffer + TAC + Alcohol + Chloride + Water 0.366
" Chloride = Hydroxylammonium chloride. b Average of three determinations.
h y d r o x y l a m m o n i u m chloride in a 25 ml total of solution. A potentiometric titration
revealed that the titanium(IV) is not reduced by the h y d r o x y a m m o n i u m chloride,
which suggest that this salt takes part in the complex formation. The Ti(IV)-TAC
complex is not formed in the presence of other reducing agents such as hydrazine
sulphate or ascorbic acid. A comparison of the spectra of titanium(IV)-TAC with
and without h y d r o x y l a m m o n i u m chloride present shows that there is no difference
between the positions of the absorbance maxima.
Accordingly, we conclude that as Ti(IV) : TAC ratio was found to be 1 : 1, a
ternary Ti(IV) : TAC : (NH2OH)x complex is formed, but the value of x could not
Table 4. Effect of the hydroxylammonium chloride [X] on the stability of the Ti(IV)-TAC system. (Ti(IV): 2.53 #g/m)
Absorbance
Table 5. Effect of the hydroxyl- ammonium chloride concentra- tion IX] on the Ti(IV)-TAC sys- tem. (Ti(IV): 2.00 #g/ml)
Time (min) ([X], 0~o) (IX], 0.40~) (I-X], 2.40~o) I-X] (~o) Absorbance (580 nm)
0 0.236 0.490 0.502 5 0.168 0.496 0.507 10 0.163 0.494 0.503 15 0.160 0.494 0.503 20 0.157 0.493 0.503 25 0.156 0.493 0.504 30 0.155 0.494 0.501 35 0.153 0.492 0.501 40 0.153 0.492 0.501 45 0.153 0.491 0.501 50 0.151 0.491 0.501 55 0.151 0.491 0.503 60 0.149 0.490 0.501 0.50 0.380 1.00 0.404 2.00 0.403 3.00 0.402 4.O0 0.404 5.00 0.405 6.00 0.405
2-(2-Thiazolylazo)-p-Cresol (TAC) 123
be determined by the conventional methods, owing to the low stability of the system
without this reactant. A 1 : 2 : 2 : 4 Ti(IV) : N H z O H : Pyrocatechol Violet : Cetyl-
t r i m e t h y l a m m o n i u m complex has been reported [-9] which lends credence to this
supposition.
Calibration Curve
The calibration curves were made under the conditions described above. The results
are shown in Table 6.
Interferences
The selectivity of the reaction was investigated by determining 12.5/~g of titanium
in the presence of various amounts of other ions. The tolerance level for an ion was
taken as the a m o u n t which caused a change of -t- 2~o in the absorbance of the chelate.
It was found that copper(II), nickel(II), iron(II), cobalt(II) and indium(III) interfere
even at 1-#g level. Magnesium, calcium, barium, strotium, aluminium and thalluim-
Table 6. Characteristics of the Ti(IV)-TAC and Fe(][I)-(TAC)2 systems
System 2(nm) e(pH 5.0) l.mole -1 cm -1 Obedience to Beer's Law Ti(IV)-TAC Fe(tI)-(TAC)z 580 9.82.103 0-125/~g 740 3.40.102 520 1.51.104 0 100 pg 580 1.08.104 0-125 #g 740 1.18.104 0-125 yg 2 = Absorption peak. e = Apparent absorption.
o.8OOA -
io6oo /
0.400 i
iO'&OO
i
OOOO /I 1
475.0
!2
540.0
roos.o
GTo.o
~ 5 . 0~)o0.o
Wavelength nm Fig. 1. Absorption spectra. 1 Ti(IV)- (TAC); 2 Fe(II)-(TAC)2
(III) did not interfere even at 200" 1 (w/w) ratio to titanium, but there was interfer-
ence from vanadium(V) (260 #g), manganese(II) (650/~g), lanthanum (850/~g), cad-
mium (260 ~g), lead (80 ~g), yttrium (40 ~g), gallium (30 ~g), zirconium (40/~g),
platinum(IV) (60 #g) and gold(III) (80 #g).
Table 7. Simultaneous determination of iron and titanium
Standard or No. of TiO2 (~o) TiO2 (~) TiO2 ( ~ o ) Fe203 (~o) Fe203 ( ~ o ) Fe203 (~o) sample c dets. present found a found b present found" found b
Bauxite NBS 10 2.78 2.73 _+ 0.04 2.74 + 0.05 5.82 5.79 +_ 0.09 5.79 _+ 0.08 C l a y l - I P T 4 0.24 0.26 _+ 0.06 0.26 _+ 0.05 1.94 2.01 ___ 0.06 2.00 ___ 0.05 C l a y 2 - I P T 6 0.54 0.59 _+ 0.06 0.60 _+ 0.06 1.46 1.49 _ 0.11 1.47 _+ 0.10 C l a y 3 - I P T 4 1.04 1.11 _ 0.02 1.08 _+ 0.02 1.28 1.32 _+ 0.08 1.30 _+ 0.09 Basalt 4 1.11 1.22 _+ 0.17 1.23 _+ 0.15 9.72 9.46 _ 0.06 9.45 _+ 0.05 Granite 4 0.26 0.25 _+ 0.02 0.25 _+ 0.03 1.62 1.84 _+ 0.10 1.85 _+ 0.09 C l a y 4 - I P T 4 1.40 1.45 _+ 0.02 1.43 _+ 0.02 1.56 1.57 _ 0.06 1.53 _+ 0.06 Clay-BA 3 0.81 0.75 _+ 0.08 0.74 _+ 0.07 7.68 7.57 _+ 0.06 7.57 _+ 0.06 Bentonite-PB 3 1.18 1.26 + 0.04 1.27 _+ 0.03 9.21 9.48 _+ 0.08 9.48 _+ 0.08 Bauxite 1 - M G 3 1.32 1.45 _ 0.04 1.44 _ 0.05 10.26 10.25 _ 0.08 10.26 + 0.09 Bauxite 2 - M G 3 1.19 1.16 _ 0.04 1.16 _+ 0.05 9.44 9.28 _+ 0.14 9.29 _+ 0.13 C e m e n t - B A 3 0.22 0.21 _+ 0.02 0.22 _ 0.03 3.33 3.18 + 0.12 3.16 _+ 0.10
a M e t h o d (a), 9 5 ~ confidence limit. b M e t h o d (b), 9 5 ~ confidence limit. c The compositions are shown in Table 8.
Table 8. C o m p o s i t i o n of standards and samples analysed
Standard and A1203 F%O3 TiO2 SiO2 C a O M g O N a 2 0 K 2 0
sample (~) (~) (~o) (~o) (~) (~) (~o) (~o)
Bauxite NBS- 55.00 5.82 2.78 6.01 0.29 0.02 0.02 0.01 U S A a Clay 1-IPT-SP a 38.40 1.94 0.24 45.10 0.07 0.14 0.01 0.85 Clay 2 - I P T - S P a 35.90 1.46 0.54 48.00 0.06 0.24 0.01 0.91 Clay 3 - I P T - S P a 29.10 1.28 1.04 55.80 0.09 0.20 0.01 0.29 Clay 4 - I P T - S P " 45.00 1.56 1.40 30.70 0.07 0.13 0.06 1.26 Basalt a 15.48 9.72 1.11 56.14 7.54 4.36 2.75 2.07 Granite -a 15.06 1.62 0.26 71.80 1.21 0.50 3.79 4.78 Clay-BA b 16.36 7.68 0.81 3.26 3.15 0.56 2.56 Bentonite-PB b 20.72 9.21 1.18 - - 0.48 2.47 0.51 1.18 Bauxite 1 - M G b 53.25 10.26 1.32 - - 0.06 0.05 0.20 0.29 Bauxite 2 - M G b 61.47 9.44 1.19 0.04 0.05 0.06 0.03 Cement-BA b 3.67 3.33 0.22 - - 53.10 1.81 0.23 0.48 a Standards. b Samples (analysed by I C P in C E P E D ) .
I P T Instituto de Pesquisa Tecnol6gica Silo Paulo; SP Brasil. C E P E D Centro de Pesquisa e Desen- volvimento da Bahia Camaqari; Ba Brasil.
2-(2-Thiazolylazo)-p-Cresol (TAC)
125
Sulphosalicylic acid, cyanide, bromide, chloride, glycine, borate, sulphate, iodide,
thiosulphate and thiourea did not interfere at 1000 : 1 (w/w) ratio to titanium, but
there was interference from EDTA (2.5 #g), NTA (2.5/~g), 1,10-phenanthroline
(125 #g), sulphide (125 #g), tartrate (400 #g), citrate (400 gg), phosphate (400 #g)
and fluoride (400 #g) in the amounts shown in the brackets.
Determination of Titanium and Iron
Although iron(II) interferes very strongly in the titanium determination, the reaction
of Fe(II) with TAC yields a complex [composition Fe(II)-(TAC)2] with absorption
peaks at 740, 580 and 520 nm. Whereas the Ti-TAC complex has only one absorp-
tion peak, at 580 nm, and practically zero absorbance at 740 nm (Fig. 1).
Simultaneous determination of titanium and iron is therefore possible, by mea-
suring the absorbances at 580 and 740 nm, and calculation based on the additive
property of the absorbances. Ideally, a correction should be included for the slight
absorbance of the Ti complex at 740 nm, but as shown in Table 7, this is not
necessary for routine analyses.
Application
The method was applied to simultaneous determination of iron and titanium in
various standards and samples (Table 8). The results (Table 7) indicate that the
accuracy and precision for the proposed method are satisfactory. To obtain higher
signals, 2-cm cuvettes were used. The method of calculation based on the measure
of the absorbance at 740 nm for the iron and at 580 nm for the titanium and iron
cannot be applied if the titanium amount is much higher than that of iron, in the
sample.
Analysis of synthetic samples showed that the method can be used for samples
with iron/titanium ratios in the range from 20 :
1 t o 1 : 3.
Acknowledgements.
